RENNER, SUSANNE S.* AND ANDRE CHANDERBALI. Department of Biology, University of Missouri-St. Louis, MO 63121 and The Missouri Botanical Garden, St. Louis, MO 63166. - What is the relationship among Hernandiaceae, Lauraceae, and Monimiaceae, and why is this question so difficult to answer?
Molecular phylogenetic studies have found that Hernandiaceae,
Lauraceae, and Monimiaceae sensu stricto form a monophyletic group,
here referred to as the HLM clade. Due to a paucity of
phylogenetically informative substitutions relationships among the
three families have remained unclear. We analyzed a total of 2,808
aligned nucleotides from a plastid intron, three spacers, and a
portion of the nuclear 26S rDNA gene in a dense sample of HLM genera,
using four outgroups with differing substitution rates. Despite
obtaining single best topologies with maximum likelihood, minimum
evolution, and parsimony approaches, family relationships remained as
poorly supported as in the previous molecular studies. Exploration of
the data indicates that varying substitution rates across lineages or
sites, insufficient taxon sampling, fast evolving outgroups, or biased
base composition are unlikely to explain the difficult reconstruction.
Exclusion of the longest-branched taxa, such as the hemiparasite
Cassytha, which is nested in Lauraceae, certain Hernandiaceae, and
outgroups, had no effect on family relationships. To resolve HLM
relationships one could add some of the basal genera sequenced here to
the existing five-gene data set of Qiu et al. (1999) to increase taxon
sampling density. Alternatively, and this is the strategy we will
pursue, one can sequence low-copy nuclear genes for key HLM genera to
sample "faster" genes.
Key words: Cassytha, Hernandiaceae, Lauraceae, Monimiaceae, substitution rate variation